Ferritic stainless steel and steel sheet for heat pipes, and heat pipe and high-temperature exhaust heat recovery system

ABSTRACT

Provided is a ferritic stainless steel for heat pipes of high-temperature exhaust heat recovery systems, which comprises, in terms of % by mass, from 16 to 32% of Cr, at most 0.03% of C, at most 0.03% of N, at most 3% of Si, at most 2% of Mn, at most 0.008% of S, from 0 to 0.3% of Al, and at least one of at most 0.7% of Nb, at most 0.3% of Ti, at most 0.5% of Zr and at most 1% of V, and optionally at least one of at most 3% of Mo, at most 3% of W, at most 3% of Cu, at most 0.1% of Y, at most 0.1% of REM (rare earth metal) and at most 0.01% of Ca, with a balance of Fe and inevitable impurities, and which satisfies at least the following formula (1), formula (2) and formula (5): 
       Cr+3(Mo+Cu)≧20  (1) 
       Cr+3(Si+Mn+Al−Ti)≧20  (2) 
       0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to stainless steel and steel sheets foruse for heat pipes for heat exchange that harness the latent heat ofevaporation of water, as well as to the heat pipe thereof and ahigh-temperature exhaust heat recovery system comprising it.

2. Background Art

Recently, a high-temperature exhaust heat recovery system is being putinto practical use for the purpose of recovering the heat of engineexhaust gas to be discharged at high temperatures in vehicle driving andrecycling it as an energy source for vehicles for the purpose ofenhancing fuel efficiency in vehicles. In general, a heat exchanger isapplied to recovery of exhaust heat, which is for heat exchange betweenexhaust heat and cooling water or any other heat medium; and as a methodof realizing efficient heat exchange, a heat exchanger with aheat-transferring means that may be referred to as a heat pipe is beingspecifically noted for use in high-temperature exhaust heat recoverysystems for vehicles, etc.

FIG. 1 shows an example of an ordinary exhaust gas passagewayconstitution that comprises a high-temperature exhaust heat recoverysystem built in the exhaust gas passageway of a vehicle. In many cases,the exhaust heat recovery system is disposed backward of the underfloorconverter as in this drawing. The heat recovered by the high-temperatureexhaust heat recovery system is effectively used for heating the enginecooling water at the start of vehicle driving and for space heating inwinter; and this contributes toward enhancing the fuel efficiency ingasoline vehicles, diesel vehicles, hybrid vehicles, etc., and savingthe batteries therein.

FIG. 2 schematically shows the principle of a heat pipe.

[0. Initial State]

The heat pipe 10 is a metallic closed vessel with pure water sealedtherein in vacuum (at most 100 Pa); and the inside of the vessel forms aliquid phase part 11 of liquid water and a space part 12.

[1. Heating/Cooling State]

When the site including the liquid phase part 11 (heating zone) of theheat pipe 10 is heated with an exhaust gas, then water in the liquidphase part 11 is actively evaporated. The evaporation is an endothermicreaction, and therefore the site can efficiently absorb the heat ofexhaust gas. Specifically, most of the heat energy of exhaust heat istransferred to the water vapor as a latent heat of evaporation(heating). When a part of the space part 12 of the heat pipe 10 (coolingzone) is cooled with cooling water or the like, then the water vaporcondenses on the inner surface of the cooled vessel of the heat pipe,and the condensed water returns back to the liquid phase part 11. Thecondensation is an exothermic reaction, and therefore the heat energycorresponding to the latent heat of the water vapor is released andtransferred to the cooling water (cooling). The cooling water thushaving received the heat energy is heated, and utilized as hot water.The cycle of heating and cooling occurs continuously, thereby realizingefficient heat exchange that harnesses the latent heat of evaporation ofwater.

Patent Reference 1 discloses a heat pipe-type high-temperature exhaustheat recovery system. FIG. 3 schematically shows the part of the system.This comprises a heating zone 22 where exhaust gas runs and a coolingzone 32 where cooling water 31 runs; and cups 23 are disposed inparallel in the heating zone 22. Between the neighboring cups 23,disposed is a heat-collecting fin 24 as brazed to the cups 23. Theexhaust gas is led to pass through the heat-collecting fin 24. The bothends of each cup 23 are connected to the upper header (vapor flow path)and bottom header (liquid reflux path), through which the heating zone22 is connected to the cooling zone 32; and the cups 23 and headers arecharged with water and sealed up after suction in vacuum as so mentionedin the above. A mode switch valve 33 capable of opening and closing theliquid reflux path is disposed at the lower part of the cooling zone 32.When water in the cup 23 is heated by an exhaust gas in an open state ofthe mode switch valve 33 (heat recovery mode), then the water circulatesin a cycle of boiling (vapor)→condensation (condensed liquid), andrecovers the exhaust heat of the exhaust gas. In many cases, the cup 23is formed to have a flattened shape so as to increase the specificsurface area thereof and to minimize as much as possible the emissionresistance of the exhaust gas; and in general, the cup is formed bypressing a stainless steel sheet having good heat resistance and goodcorrosion resistance. In many cases, the temperature of the exhaust gasto be fed to the heating zone 22 is elevated owing to the catalyticeffect of the converter, and is often 800° C. or higher. The materialtemperature of the heating zone 22 is the highest when the mode switchvalve 33 is in a closed state (heat shutoff mode), and is presumed toreach in a temperature range of from 600 to 900° C.

In that manner, the heat pipe is heated up to a temperature range of900° C. or so, and therefore must be formed of a material excellent inhigh-temperature strength (creeping resistance, high-temperature fatigueresistance, heat fatigue resistance) and also excellent inhigh-temperature oxidation resistance. In addition, it requiresexcellent workability, weldability and brazability. Importantly, inaddition, it must be inexpensive. Totaling these requirements, atpresent, it is considered that stainless steel is the most suitable forheat pipe material. Patent Reference 2 discloses a heat pipe formed of astainless steel which contains 14-27% by weight of Cr.

Patent Reference 1: JP-A 2007-327719

Patent Reference 2: JP-A 7-243784

However, a heat pipe formed of stainless steel may have a large quantityof hydrogen generated inside it in the early stage of driving. As aresult of various investigations, it has been clarified that the innerpressure may be often over 1 MPa owing to the generated hydrogen. Insuch a case, the heat pipe shall be a pressure vessel to be under severelegal controls. For handy utilization as an exhaust heat recovery systemof high popularity, it is desirable to construct a high-temperatureexhaust heat recovery system in which the inner pressure of the heatpipe is not over 1 MPa.

In addition, the hydrogen generation inside the heat pipe is a riskfactor of lowering the heat transfer efficiency to cooling water and isadditionally a risk factor of imparting excessive stress to the system;and therefore this may be a remote cause of system damage and heavydisasters.

A ferritic stainless steel has a smaller thermal expansion coefficientas compared with austenitic steels, and is therefore advantageous inpoint of the thermal fatigue resistance in cycles of heating andcooling. In addition, a ferritic stainless steel has a smaller hydrogendiffusion coefficient as compared with austenitic steels, and istherefore advantageous for discharging hydrogen generated inside avessel out of the system. Further, a ferritic stainless steel isgenerally more inexpensive than austenitic steels. On the negative side,a ferritic stainless steel is generally inferior to austenitic steels inpoint of high-temperature oxidation resistance, high-temperaturestrength, corrosion resistance, etc.

SUMMARY OF THE INVENTION

An object of the present invention is to develop a ferritic stainlesssteel of a type of ferritic steels naturally having the above-mentionedadvantages, which has the property of stably preventing the pressureincrease to be caused by hydrogen generation in use thereof for vesselsconstituting a heat pipe, which has high-temperature oxidationresistance and high-temperature strength enough for use thereof in atemperature range of from 600 to 900° C., and which has corrosionresistance and Ni brazability suitable for heat pipes; and to provide atechnique capable of constructing a heat pipe-type exhaust heat recoverysystem of high popularity.

To attain the above-mentioned object, the invention provides a ferriticstainless steel for heat pipes of high-temperature exhaust heat recoverysystems, which comprises, in terms of % by mass, from 16 to 32% of Cr,at most 0.03% of C, at most 0.03% of N, at most 3% of Si, at most 2% ofMn, at most 0.008% of S, from 0 (no addition) to 0.3% of Al, and atleast one of at most 0.7% of Nb, at most 0.3% of Ti, at most 0.5% of Zrand at most 1% of V, with a balance of Fe and inevitable impurities, andwhich satisfies all of the following formula (1), formula (2) andformula (5) and satisfies at least one of the following formula (3) andformula (4). The invention also provides the ferritic stainless steelfurther containing at least one of at most 3% of Mo, at most 3% of W andat most 3% of Cu. The ferritic stainless steel may further contain atleast one of at most 0.1% of Y, at most 0.1% of REM (rare earth metal)and at most 0.01% of Ca.

Cr+3(Mo+Cu)≧20  (1)

Cr+3(Si+Mn+Al−Ti)≧20  (2)

Ti+Al≦0.5  (3)

Nb≧Ti+Al  (4)

0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5)

In formulae (1) to (5), the site of the atomic symbol is substitutedwith the value of the content of the corresponding element in terms of %by mass, and the site of the element not added to the steel issubstituted with 0 (zero).

The invention also provides a ferritic stainless steel sheet for heatpipes, which is formed of the above-mentioned steel and which has apolish-finished surface at a polish count of from #100 to #800 asdefined in JIS R6001:1998, or has a polish-finished surface at HL asdefined in Table 14 in JIS G4305:2005.

The invention also provides a heat pipe constructed by vacuuming avessel formed of a steel material of the above-mentioned steel having athickness of from 0.5 to 1 mm, followed by introducing water thereintoand sealing it up to thereby form a liquid phase part and a space parttherein. Preferably, the vessel has, as the inner surface thereof, apolish-finished surface at a polish count of from #100 to #800 asdefined in JIS R6001:1998, or a polish-finished surface at HL as definedin Table 14 in JIS G4305:2005. The invention further provides ahigh-temperature exhaust heat recovery system having the above-mentionedheat pipe disposed in a site where the material temperature reaches in arange of from 600 to 900° C.

According to the invention, the increase in the inner pressure of astainless steel-made heat pipe, which is caused by hydrogen generationin the heat pipe and is problematic therein, can be significantlyrelieved. Using the heat pipe can realize a high-temperature exhaustheat recovery system of high popularity at a low cost, thereforecontributing toward further popularization of exhaust heat recoverysystems in vehicles, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of an exhaust gaspassageway constitution that comprises a high-temperature exhaust heatrecovery system built in the exhaust gas passageway of a vehicle.

FIG. 2 is a conceptual view schematically showing the principle of aheat pipe.

FIG. 3 is a schematic view showing an example of a conventionalhigh-temperature exhaust heat recovery system.

FIG. 4 is a schematic view showing the constitution of a hydrogengeneration test apparatus.

FIG. 5 is a view schematically showing the member constitution of a testbody for hydrogen permeation test.

FIG. 6 is a view schematically showing the constitution of a hydrogenpermeation test apparatus.

FIG. 7 is a view explaining a Corrosion test method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For preventing the pressure increase to be caused by hydrogen generationin a heat pipe, first the mechanism thereof must be known. Aftervacuumed, a heat pipe is charged with pure water and sealed up, andtherefore, only pure water and metal of the heat pipe (vessel) itselfexist inside it. The hydrogen generation inside the heat pipe is causedby oxidation of the metal constituting the heat pipe (vessel) with water(water vapor). The reaction may be considered essentially as follows:

xM+yH₂O→M_(x)O_(y) +yH₂↑

where M means a metal, and x and y each are a coefficient.

The present inventor's investigations have clarified that the hydrogengenerated inside a heat pipe penetrates through the heat pipe material(vessel) to go outside in some degree. Accordingly, for reducing thepressure increase to be caused by the hydrogen generation inside theheat pipe, it is effective to use any of the following two materials inconstituting the heat pipe.

(i) Material which hardly generates hydrogen.(ii) Material through which hydrogen readily penetrates.

Different frost austenitic steels, ferritic steels are naturally almostgood in point of (ii). Accordingly, it is important to improve ferriticsteels in point of (i).

Regarding the above (i), it is necessary to enhance the water vaporoxidation resistance of stainless steel that constitutes a heat pipe.When coated with a dense Cr₂O₃ film on the surface thereof, stainlesssteel may have enhanced water vapor oxidation resistance. As the meansfor it, the present inventors have found that employing the followingmethod [1] is extremely effective for a ferritic stainless steel, andemploying the following method [2] or [3] is more preferred. In theinvention, the following [1] is employed, and if desired, any one orboth of the following [2] and [3] are employed.

[1] The steel composition comprises Cr, Si and Mn each in the rangementioned below, and satisfies the following formula (2):

Cr+3(Si+Mn+Al−Ti)≧20  (2)

For preventing the hydrogen generation in use of heat pipes, it iseffective to positively add to a ferritic stainless steel, Cr, Si and Mnhaving the effect of inhibiting the growth of oxidation scale at 600 to900° C., each in the range mentioned below. Addition of Al is alsoeffective. However, when the steel contains Ti, it has been found thatthe hydrogen generation increases owing to oxidation of Ti. Ti may beadded as an element for fixing C and N in steel, as mentioned below, andtherefore in the invention, it is especially important to control the Ticontent of the ferritic stainless steel of the invention. As a result ofvarious investigations, the inventors have found that theabove-mentioned formula (2) must be satisfied for reducing hydrogengeneration.

[2] The steel composition contains Y, REM (rare earth metal) and Ca eachin the range mentioned below.

These elements have the effect of enhancing the high-temperatureoxidation resistance of a ferritic stainless steel, and are effectivefor reducing hydrogen generation.

[3] The steel sheet has a polish-finished surface.

A ferritic stainless steel may readily form a poorly protectiveoxidation scale (red scale) that comprises Fe₂O₃ as the main ingredientthereof, in a low-oxygen high-moisture environment at 500 to 600° C. Aheat pipe is often exposed to that environment in a cycle of heating itup to the highest service temperature thereof and cooling it to ordinarytemperatures, in which the oxidation reaction of forming red scale maybe often problematic as generating a large quantity of hydrogen. Thepresent inventors have found that, for this problem, it is extremelyeffective to previously impart working strain to the surface layer ofthe ferritic stainless steel sheet that is exposed to theabove-mentioned environment. According to this, Cr, Si, Mn and Al couldreadily diffuse into the surface layer of the steel sheet in a middletemperature range of from 500 to 600° C., whereby a protective denseoxidation film could be rapidly formed to prevent the formation of redscale on the steel sheet.

For forming working strain in the surface layer, there may be employed amethod of shot blasting, polishing or the like; but in the invention,polishing is employed suitable for large-scale mass-production. Thepresent inventors' investigations have clarified that strainintroduction to a depth of 50 μm or so from the surface layer may beenough for the purpose. For this, the steel sheet may be so processed asto have a polish-finished surface at a polish count of from #100 to #800as defined in JIS R6001:1998, or a polish-finished surface at HL asdefined in Table 14 in JIS G4305:2005.

The steel to which the invention is directed is preferably a steel(ferritic single-phase steel) of which the composition is so designed asnot to form as much as possible an austenitic phase at a hightemperature. A steel having a composition that may readily form anaustenitic phase at a high temperature is undesirable in that, when aCr-deficient layer is formed in the matrix just below the oxidation filmthat covers the steel, then the Cr-deficient part may form locally anaustenitic phase at a high temperature and may be therefore a bar todiffusion of hydrogen. In the steel of the type, in addition, when theaustenitic phase formed at a high temperature is transformed into amartensitic phase in cooling, then the steel may have still anotherproblem of fatigue strength reduction at ordinary temperature and athigh temperatures owing to hydrogen embrittlement.

The alloying elements are described below. In this description, “%” forthe alloying elements is “by mass” unless otherwise specificallyindicated.

Cr is an important alloying ingredient for imparting the necessarycorrosion resistance and oxidation resistance to the stainless steel. Inorder that the steel can secure water vapor oxidation resistance at 600to 900° C., it requires at least 16% of Cr, and must satisfy thefollowing formula (1):

Cr+3(Mo+Cu)≧20  (1)

For example, when Mo and Cu are not added to the steel, then 0 (zero) issubstituted into the sites of Mo and Cu, and Cr alone must account forat least 20% in the formula (1).

When the Cr content is higher, then the ferrite stability is higher,therefore retarding Cr-deficient layer formation and retardingaustenitic transformation in the Cr-deficient layer, if formed. Morepreferably, the Cr content is at least 13%, and even more preferably atleast 20% especially when the above-mentioned polishing or preliminaryoxidation treatment is not applied to the steel. On the other hand,however, the Cr content of more than 32%, if any, may greatly worsen theworkability and the embrittlement resistance at 475° C. of the steel.Accordingly, the Cr content is within a range of at most 32%.

C and N are elements of enhancing the high-temperature strength,especially the creeping resistance of the steel; however, when they arein the steel as solid solution elements therein, then they may traphydrogen in the steel to form methane and ammonia, thereby causing arisk factor of reducing the high-temperature strength, the toughness andthe hydrogen permeability of the steel. In addition, since C and N areaustenite-forming elements, they may also cause a risk factor of formingan austenitic phase at a high temperature. In this case, the steel mayhave a problem of hydrogen permeability reduction in the Cr-deficientlayer and fatigue strength reduction owing to hydrogen embrittlement, asso mentioned in the above. Accordingly, the steel preferably has a lowerC content and a lower N content as much as possible. As a result ofvarious investigations, C and N are both acceptable in an amount of upto 0.03% each. However, for sufficiently lowering the sum total of solidsolution C and solid solution N (hereinafter referred to as “amount ofsolid solution C+N”) in the steel, it is important that the steelsatisfies the following formula (5) in relation to the content of Nb,Ti, Zr and V therein that readily bond to C and N:

0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5)

In the formula (5), the left-hand member is an index of indicating theamount of solid solution C+N in the ferritic stainless steel having thecomposition of the invention. When the steel does not satisfy theformula (5), the durability of the heat pipe formed of the steel maygreatly lower since the thermal fatigue resistance at high temperaturesof the steel is poor.

Si is an element of enhancing the oxidation resistance and the red scaleresistance of the steel, therefore acting to prevent hydrogengeneration. In addition, this is an element of having an effect offerrite stabilization. Especially effectively, the Si content is atleast 0.1%; however, too much Si, if any, may worsen the hot ductilityof the steel and may cause formation of faults in the steel surface,therefore causing a risk factor of greatly worsening the producibilityand the weldability of the steel. Accordingly; the Si content is limitedto fall in a range of at most 3%.

Mn is an element of enhancing the scale peeling resistance of thestainless steel, and is effective for preventing the pressure increaseto be caused by the hydrogen generation inside the heat pipe formed ofthe steel. However, too much Mn, if any, may detract from the toughnessof the steel; and therefore, the Mn content is limited to be at most 2%.

S is an ingredient that may have some negative influences on the hotworkability and the welding-resistant high-temperature crackingresistance of the steel, and may be a starting point for abnormaloxidation in the steel. Therefore, the S content is preferably as low aspossible; and its uppermost limit is defined to be at most 0.008% bymass. More preferably, the S content is at most 0.005% by mass.

Nb, Ti, Zr and V act to fix C and N to thereby reduce the amount ofsolid solution C and solid solution N in the steel. In addition, theyalso act to enhance the high-temperature strength of the ferriticstainless steel through precipitation enhancement by fine dispersion oftheir carbides and nitrides in the steel. Al has an effect of enhancingthe oxidation resistance of the steel and an effect of reducing thehydrogen generation by the steel. To attain these effects in theinvention, at least one of at most 0.7% of Nb, at most 0.3% of Ti, atmost 0.5% of Zr and at most 1% of V is added to the steel. Inparticular, more preferably, at least one of from 0.001 to 0.7% of Nb,from 0.005 to 0.3% of Ti, from 0.001 to 0.5% of Zr and from 0.05 to 1%of V is added. Al is optionally added to the steel in an amount of atmost 0.3%. In case where Al is added, then its content is moreeffectively in a range of from 0.03 to 0.3%. However, special attentionshould be paid to adding Ti and Al. One reason is that Ti may be a riskfactor of hydrogen generation as so mentioned in the above. This problemmay be solved by composition control to satisfy the above formula (2).Another reason is that Ti and Al may be a risk factor of worsening thebrazability of the steel. Most heat pipe members are put into practicaluse by brazing; however, when the content of Ti and Al in the ferriticstainless steel is high, Ti and Al may form an oxidation film under heatin brazing even under a low oxygen partial pressure condition, thereforedetracting from the brazability of the steel (especially the wettabilityof the steel to a brazing material). As a result of detailedinvestigations, this problem can be solved by composition control tosatisfy at least one of the following formula (3) and formula (4):

Ti+Al≦0.05  (3)

Nb≧Ti+Al  (4)

As so mentioned in the above, the steel must contain at least one of Nb,Ti, Zr and V satisfying the formula (5) in order to fully reduce theamount of solid solution C+N therein.

Mo, W and Cu are all elements of enhancing the high-temperature strengthof the stainless steel through solid solution reinforcement. In theinvention, a method of adding a large amount of C and N for enhancingthe strength of the steel could not be employed, and therefore, in casewhere the strength of the steel is emphasized, preferably, at least oneof Mo, W and Cu is added to the steel. In addition, Mo and Cu especiallyhave an effect of retarding the generation and growth of pittingcorrosion of the dew point corrosion to be caused by the dewcondensation in exhaust gas. Adding at least one of Mo and Cu maybroaden the range of the Cr content (that is, may lower the lowermostlimit thereof) necessary for satisfying the formula (1). In case whereat least one of Mo, W and Cu is added, preferably, they are socontrolled that the Mo content is at most 3%, the W content is at most3% and the Cu content is at most 3%. More effectively, the content ofMo, W and Cu is at least 0.1% each.

Y, REM (rare earth metal) and Ca have an effect of fixing S that isharmful to the scale peeling resistance of the steel, thereby toprohibit thickening S in the scale/base interface, and have an effect ofreducing the defect density in the oxidation film to thereby reinforcethe oxidation film, whereby the oxidation resistance of the steel can begreatly enhanced. Accordingly, in the invention, at least one of thesemay be optionally added to the steel. However, too much addition ofthose elements may not only harden the steel material too much but alsocause surface faults in production of the steel, therefore resulting inthe increase in the production cost. As a result of variousinvestigations, in case where these elements are added, they arepreferably so controlled that the Y content is at most 0.1%, the REMcontent is at most 0.1%, and the Ca content is at most 0.01%. Moreeffectively, at least one of from 0.0005 to 0.1% of Y, from 0.0005 to0.1% of REM and from 0.0005 to 0.01% of Ca is added to the steel.

In melt-producing the stainless steel, Ni, P, B, Mg and others may mixtherein, for example, from the starting materials and from the depositsadhering to the wall surface of the smelting equipment. The acceptablerange of Ni is up to 0.6%, that of P is up to 0.05%, that of B is up to0.01%, and that of Mg is up to 0.005%.

In producing a heat pipe by the use of the ferritic stainless steel,first a stainless steel sheet having the above-mentioned predeterminedcomposition is prepared according to a production process for anordinary ferritic stainless steel sheet; then a sheet piece cut out ofthe steel sheet is shaped and welded into a vessel, and thereafter thevessel is vacuumed and then charged with pure water. Another method isalso employable wherein a welded steel pipe is formed from the steelsheet, or a seamless steel pipe is formed from the steel billet and thesteel pipes are worked into a vessel. The technique of “charging withpure water” means that the vessel into which pure water has been put issealed up by welding or meltdown fusing. When the wall thickness of thevessel is too thin, the vessel could hardly have sufficient durability.On the other hand, when too thick, then the hydrogen diffusion distancethrough the vessel wall may be long since hydrogen must penetratethrough the thick wall, and this is disadvantageous for reducing theinner pressure of the vessel. With the increase in the heat transferdistance through the steel material, the thermal conductivity resistanceof the steel material may increase, which, however, is disadvantageousfor efficient thermal exchange based on the evaporation latent heat ofsteam. As a result of various investigations, the wall thickness of theheat pipe is preferably within a range of from 0.5 to 1 mm.

Example 1

Steels shown in Table 1 (30 kg each) were melt-produced in a vacuumsmelting furnace, and hot-rolled, annealed, pickled, cold-rolled andfinish-annealed according to an ordinary ferritic stainless steel sheetproduction method, thereby producing cold-rolled annealed pickled steelmaterials (sample materials) having a thickness of 1.0 mm, for which thepickling finish was No. 2D as defined in Table 14 in JIS G4305:2005.

TABLE 1 Chemical Composition (mas. %) Classification No. C Si Mn P S NiCr Mo Cu N Nb Ti Al others Samples of A1 0.02 0.30 1.00 0.025 0.002 0.1118.3 2.05 0.18 0.01 0.65 — — — the invention A2 0.01 0.19 0.22 0.0330.001 0.18 21.9 0.98 0.12 0.01 0.27 0.17 0.08 — A3 0.01 0.25 0.35 0.0300.002 0.15 18.2 1.07 0.08 0.01 0.35 — — — A4 0.01 0.22 0.31 0.027 0.0020.18 18.5 1.01 0.13 0.01 — — — Zr: 0.24 A5 0.01 0.27 0.42 0.031 0.0010.08 18.4 0.95 — 0.01 0.10 — — V: 0.23 A6 0.01 0.55 0.26 0.028 0.0010.09 18.5 — 0.49 0.02 0.45 0.04 — Ca: 0.005 A7 0.01 0.31 0.22 0.0210.003 0.11 20.2 2.01 1.52 0.01 0.45 — — W: 1.24 A8 0.02 0.22 0.31 0.0220.001 0.09 20.5 0.23 — 0.01 0.42 — — REM: 0.02 A9 0.01 0.18 0.25 0.0290.001 0.07 23.9 — — 0.01 0.05 — — Y: 0.02 A10 0.01 0.22 0.28 0.028 0.0010.11 29.9 1.95 0.08 0.01 0.35 0.19 0.11 — Comparative B1 0.01 0.45 0.350.030 0.001 0.15 17.9 1.01 — 0.01 0.04 0.35 — — Samples B2 0.01 0.290.22 0.024 0.002 0.03 17.3 0.05 0.07 0.01 — 0.41 — — B3 0.01 1.52 0.310.022 0.001 0.12 12.2 — — 0.01 — 0.15 0.98 — B4 0.02 0.22 0.25 0.0250.001 0.15 18.1 0.05 0.07 0.01 — 0.13 2.97 — B5 0.03 0.95 1.10 0.0330.001 0.15 14.0 — 0.07 0.01 0.44 — — — B6 0.06 0.55 0.25 0.033 0.0030.15 16.9 0.09 — 0.02 0.01 — — — B7 0.04 0.22 0.45 0.035 0.002 0.22 19.2— — 0.02 0.05 — — — left-hand member left-hand member left-hand memberClassification No. of formula (1) of formula (2) Ti + Al of formula (5)Samples of A1 24.99 22.20 0.0 0.004 the invention A2 25.20 22.86 0.250.003 A3 21.65 20.00 0.00 0.005 A4 21.92 20.09 0.00 0.007 A5 21.25 20.470.00 0.004 A6 19.97 20.81 0.04 0.005 A7 30.79 21.79 0.00 0.004 A8 21.1922.09 0.00 0.006 A9 23.90 25.19 0.00 0.031 A10 35.99 31.16 0.30 0.003Comparative B1 20.93 19.25 0.35 0.003 Samples B2 17.66 17.60 0.41 0.003B3 12.20 20.18 1.13 0.006 B4 18.46 28.03 3.10 0.010 B5 14.21 20.15 0.000.008 B6 17.17 19.30 0.00 0.593 B7 19.20 21.21 0.00 0.090

<<Hydrogen Generation Test>>

Hydrogen generation test pieces of 10 mm×50 mm×1.0 mm each were cut outof each sample material. These were grouped into two; and the sheetsurface of those in one group was the No. 2D pickle-finished surface assuch, while that in the other group was a #400 dry polish-finishedsurface. The cut edges of all the test pieces were all #600 drypolish-finished ones.

FIG. 4 schematically shows the constitution of the hydrogen generationtest apparatus used herein. A test piece is put into the quartz tube,and this is set in an electric furnace. The quartz tube is insulatedfrom the outside world by a quartz cover fitting thereto. However, asheathed thermocouple housed in a quartz protective tube is insertedinto the quartz tube so as to measure the temperature around the sample.A Pyrex® tube is connected to the quartz tube, and the Pyrex tube isconnected to a vacuum pump and a pure water tank. The quartz tube, thequartz cover, the Pyrex tube and the quartz protective tube are allconnected to each other by lapping, and the airtightness of the lappedpart is suitably increased by a vacuum grease. At the beginning, all thevalves (A to D) are shut.

First, only the valve D is opened at normal temperature. In this step,the tube between the valves C and D is filled with about 30 mL of purewater. Next, the valve D is again shut. Next, the vacuum pump is driven,the valve B is opened and the quartz tube is vacuumed, and then thevalve A is opened and the pressure is measured. After vacuuming to 1 Pa,the valve B is shut. Next, the valve C is opened. In this stage, thepure water remaining between the valves C and D is led into the quartztube via the Pyrex tube. As vigorously sucked into the vacuum, almostall the water having remained in the tube C-D reaches near the testpiece.

Next, the inside of the quartz tube is heated by the electric furnace.Monitored with the sheathed thermocouple, the temperature inside thequartz tube is kept at 600° C. or 800° C. The pressure change during thetest where the temperature is kept at 600° C. or 800° C. for 10 hours ismonitored, and from the pressure change at the time after 10 hours, thehydrogen generation rate from the sample surface per the unit time andthe unit area thereof is computed. The samples from which the hydrogengeneration rate at the time after 10 hours is at most 1.0×10⁻⁶mol/(h·cm²) are determined as those having the property of reducing theinner pressure increase to at most 10 kPa in a real heat pipe.Accordingly, the samples from which the hydrogen generation rate at thetime after 10 hours is at most 1.0×10⁻⁶ mol/(h·cm²) are evaluated asgood (as effective for reducing hydrogen generation), while those fromwhich the hydrogen generation rate is over the range are as bad (asineffective for reducing hydrogen generation). The results are shown inTable 2.

For reference, in Table 2, the data of the hydrogen generation rate ofeach sample at the time after kept at 600° C. for 1 hour are also shownunder the same evaluation standard as above.

<<Hydrogen Permeation Test, Brazability Evaluation>>

FIG. 5 schematically shows the member constitution of a test body forhydrogen permeation test. The members formed of a test material areplates (two) and cups (four). The cups are formed by pressing; and theouter shape dimension thereof is about 100 mm (length)×30 mm (width)×5mm (height). In FIG. 5, the width and the height of the cup are drawn asexaggerated, as compared with the length thereof. Two holes are made inthe bottom of each cup, and the inner space of one cup communicates withthat of the adjacent cup via linking pipes attached to the holes. Thelinking pipe is made of a ½-inch pipe of SUS310S by flattening it. Thedistance between the two cups linked via the linking pipes is 8 mm. Thecenter two cups are integrated to be a bag. The side cup is covered withthe plate. A hole is made in the center of the plate to be fitted to oneend of the test body, and hydrogen gas is introduced into the test bodythrough the hole. These members are braded and soldered with a Ni-basedbrazing material (BNi-5), and the test body is thus constructed. Thebraded part is airtightly sealed up to prevent gas leakage through it.The brazing is attained in a vacuum furnace, and the vacuum brazingcondition is as follows: The pressure is 1 Pa, the brazing temperatureis 1175° C., the heating time from ordinary temperature up to thebrazing temperature is 2 hours, and the soaking time at the brazingtemperature is 30 minutes.

FIG. 6 schematically shows the constitution of a hydrogen permeationtest apparatus. The test body in FIG. 6 is drawn as exaggerated in theheight direction (in the lamination direction) thereof. The test body isset in an electric furnace. Via the ¼ inch tube of SUS316 braded to thetest body, hydrogen gas is introduced into the inside of the test bodyfrom a hydrogen generation unit. First, the inside of the test body isvacuumed to 1 Pa by the vacuum pump. While the valve Y is shut, thevalve X is opened and hydrogen gas is introduced into the test body fromthe hydrogen generation unit, and when the pressure inside the test bodybecomes higher than 120 kPa, the valve X is shut. Accordingly, hydrogengas having a hydrogen partial pressure of more than about 120 kPa issealed up inside the test body. Next, the electric furnace is heated andthe test body therein is kept at 800° C. While kept at 800° C., thepressure change inside the test body is monitored; and from the pressurechange at the time at which the pressure is 100 kPa (or that is, at thetime at which the hydrogen partial pressure is about 100 kPa), thehydrogen permeation rate is computed. Then, the value of the hydrogenpermeation rate is divided by the surface area of the part exposed tothe inner atmosphere of the four cups and the two plates composed of thetest material, thereby giving the hydrogen permeation rate of the testmaterial per the unit time and per the unit area thereof. Hydrogen maypermeate into the other members than those of the test material existinginside the surface (part of ¼ inch tube and linking pipe); however, thesurface area of the other members than those of the test material issufficiently smaller than the surface area of the test material, and thehydrogen permeation through the other members than the test material canbe ignored.

The samples of which the hydrogen permeation rate thus measured in themanner as above is at least 1.0×10⁻⁹ mol/(h·cm²·Pa^(1/2)) are determinedas those having the property of reducing the inner pressure increase toat most 10 kPa in a real heat pipe, based on the condition that thehydrogen generation rate from the samples, as measured according to theabove-mentioned test method, is at most 1.0×10⁻⁶ mol/(h·cm²).Accordingly, the samples of which the hydrogen permeation rate is atleast 1.0×10⁻⁹ mol/(h·cm²·Pa^(1/2)) are evaluated as good (as effectivefor increasing hydrogen permeation), while those of which the hydrogenpermeation rate is lower than the range are as bad (as ineffective forincreasing hydrogen permeation).

The samples with brazing failure could not have a predetermined vacuumdegree owing to vapor leakage through the braded part in vacuuming, andtherefore could not be tested in the hydrogen permeation test.Accordingly, the brazability of the samples is evaluated as follows: Thesamples that could have a predetermined vacuum degree are as good (ashaving good brazability); and the others are evaluated as bad (as havingpoor brazability).

The results are shown in Table 2.

<<Corrosion Test>>

The corrosion test is attained according to a test in which an exhaustgas dew condensation environment is simulated; and an outline of thetest method is shown in FIG. 7. Specifically, a test piece of 25 mm×70mm is cut out of each test material; its surface is #400 drypolish-finished and its cut edges are #600 dry polish-finished; and thisis heat-treated at 800° C. for 10 hours to prepare a corrosion testpiece. The composition of simulated condensed water is shown in theappendix table in FIG. 7. The corrosion test piece is set between avertical heating furnace and a water tank filled with simulatedcondensed water put below it, in such a manner that it can bereciprocated up and down between the two, and exposed to 50 heating anddipping cycles, in which one cycle comprises “keeping in furnace at 350°C. for 6 minutes (including soaking time of about 1 minute)→coolingoutside the furnace for 7 minutes (the temperature of the test piece,not higher than 100° C.)→dipping the lower part, 20 mm of the test piecein the simulated condensed water for 1 minute→drying in air for 10minutes (for condensation of the ingredients in the liquid)”. Then, thetest piece is kept in a thermo-hygrostat at 30° C. and 80% RH for 2000hours, and after the soaking therein, the maximum corrosion depth of thecorroded pores formed in the test piece is measured according to a focaldepth method using an optical microscope. In case where the maximumcorrosion depth in this test method is more than 0.8 mm, there may be apossibility that the cup part of the heat pipe formed of the test piecemay be corroded to have a through-hole after exposed to a vehicleexhaust gas dew condensation atmosphere for 15 years, even though aplate thickness of 1 mm that is the largest thickness planned for heatpipes for vehicles is assumed. Accordingly, the samples of which themaximum corrosion depth in this test is at most 0.8 mm are evaluated asgood (as having good corrosion resistance), and the others are as bad(as having poor corrosion resistance).

The results are shown in Table 2.

TABLE 2 Hydrogen Generation Rate (reference) Hydrogen 800° C., 600° C.,Hydrogen Corrosion Generation Rate after 10 hr after 10 hr PermeationTest 600° C., after 1 hr Classification No. #400 2D #400 2D BrazabilityRate 800° C. 2000 hr #400 2D Samples of A1 good good good good good goodgood good bad the invention A2 good good good good good good good goodbad A3 good good good good good good good good bad A4 good good goodgood good good good good bad A5 good good good good good good good goodbad A6 good good good good good good good good bad A7 good good goodgood good good good good bad A8 good good good good good good good goodbad A9 good good good good good good good good bad A10 good good goodgood good good good good bad Comparative B1 bad bad good good good(immeasurable) good good bad Samples 82 bad bad good good good(immeasurable) bad good bad B3 good good good good good (immeasurable)bad good bad B4 good good good good good (immeasurable) bad good bad B5good good good good good good bad good bad B6 bad bad good good goodgood bad good bad B7 good good good good good good bad good bad

As known from Table 2, all the samples of the invention had good resultsin all the test items. In particular, it is considered that the sampleswith strain introduced into the surface layer thereof by polishing couldrapidly form a protective oxidation scale in the initial stage ofheating in a middle temperature range of around 600° C., and thehydrogen generation rate from them decreased at the time after 1 hour at600° C. Accordingly, these samples could exhibit a more stable hydrogengeneration-preventing effect in the initial stage of starting the use ofheat pipes.

As opposed to these, the samples B2 to B7 not satisfying the formula (1)were poor in the corrosion resistance; and the samples B1, B2 and B6 notsatisfying the formula (2) had a large hydrogen generation rate at 800°C. falling within the maximum ultimate temperature range of heat pipes.The samples B1 to B4 not satisfying the formula (3) and the formula (4)were poor in brazability.

Example 2 Heating/Cooling Cycle Durability Test

Using a cold-rolled annealed steel sheet (#400 dry polish-finished steelsheet) having a thickness of 0.8 mm of the samples A1 to A3, B6 and B7in Table 1, the heat pipe (cup 23) of a high-temperature exhaust heatrecovery system as in FIG. 3 was constructed. The heating methodemployed herein comprises introducing a high-temperature gas into theheat-collecting fin 24 from an external gas burner. The system wasexposed to a test of heating/cooling 2000 cycles, in which one cyclecomprises “1. heating with coolant (water) circulation for 5 minutes→2.further heating for 5 minutes with coolant circulation stopped→3.coolant circulation and heating stopped for 5 minutes”. The temperatureof the heat pipe was about 400° C. in the step 1 of heating with coolantcirculation, about 800° C. in the step 2 of heating with coolantcirculation stopped, and from 100 to 200° C. in the cooling step 3; andin this cycle, the system is not corroded by dew condensation. After theheating/cooling cycle test, the system was dismantled, and the membersof the cup 23 were color-checked for the presence or absence of damages.As a result, no damage was detected in the cup 23 formed of the sampleA1 to A3 of the invention. As opposed to this, the cup 23 formed of thesample B6 or B7 not satisfying the formula (5) had defects runningthrough the sheet owing to the large content of solid solution C+Ntherein.

1. A ferritic stainless steel for heat pipes of high-temperature exhaustheat recovery systems, which comprises, in terms of % by mass, from 16to 32% of Cr, at most 0.03% of C, at most 0.03% of N, at most 3% of Si,at most 2% of Mn, at most 0.008% of S, from 0 (no addition) to 0.3% ofAl, and at least one of at most 0.7% of Nb, at most 0.3% of Ti, at most0.5% of Zr and at most 1% of V, with a balance of Fe and inevitableimpurities, and which satisfies all of the following formula (1),formula (2) and formula (5) and satisfies at least one of the followingformula (3) and formula (4):Cr+3(Mo+Cu)≧20  (1)Cr+3(Si+Mn+Al−Ti)≧20  (2)Ti+Al≦0.05  (3)Nb≧Ti+Al  (4)0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5), wherein the site of theatomic symbol is substituted with the value of the content of thecorresponding element in terms of % by mass, and the site of the elementnot added to the steel is substituted with 0 (zero) in formulae (1) to(5).
 2. The ferritic stainless steel for heat pipes as claimed in claim1, which further contains at least one of at most 3% of Mo, at most 3%of W and at most 3% of Cu.
 3. The ferritic stainless steel for heatpipes as claimed in claim 1, which further contains at least one of atmost 0.1% of Y, at most 0.1% of REM (rare earth metal) and at most 0.01%of Ca.
 4. A ferritic stainless steel sheet for heat pipes, which isformed of the steel of claim 1 and which has a polish-finished surfaceat a polish count of from #100 to #800 as defined in JIS R6001:1998, orhas a polish-finished surface at HL as defined in Table 14 in JISG4305:2005.
 5. A heat pipe for high-temperature exhaust heat recoverysystems, as constructed by vacuuming a vessel formed of a steel materialof the steel of claim 1 and having a thickness of from 0.5 to 1 mm,followed by introducing water thereinto and sealing it up to therebyform a liquid phase part and a space part therein.
 6. A heat pipe forhigh-temperature exhaust heat recovery systems, as constructed byvacuuming a vessel formed of a steel material of the steel of claim 1and having a thickness of from 0.5 to 1 mm and having, as the innersurface thereof, a polish-finished surface at a polish count of from#100 to #800 as defined in JIS R6001:1998 or a polish-finished surfaceat HL as defined in Table 14 in JIS G4305:2005, followed by introducingwater thereinto and sealing it up to thereby form a liquid phase partand a space part therein.
 7. A high-temperature exhaust heat recoverysystem having the heat pipe of claim 5 as disposed in a site where thematerial temperature reaches from 600 to 900° C.